This invention relates to relief valves for pistons which serve to position equipment or levers.
In many equipment structures, including agricultural implements, hydraulic cylinders are used to raise and lower or otherwise move members of the machine and to retain the members in a position until the hydraulic cylinder is actuated to move the member to a new position. When hydraulic fluid is pumped into the hydraulic cylinder and the piston has traveled fully to the end of the cylinder, the hydraulic pump supplying fluid to the cylinder continues to apply pressure to the fluid and must be stopped to avoid an over pressure condition which stresses and damages the cylinder end cap and head gland. In some existing systems, timer mechanisms are operable with the hydraulic pump to shut the pump off after the estimated time it takes for the piston to travel to an end of the cylinder. At best, this only approximates efficient operation.
In other prior art devices, a transducer is provided on the hydraulic cylinder to sense when a predetermined pressure is attained in the fluid input line. The transducer either sends a signal to the pump control to stop pumping, or to a switch valve to bypass the input line so that over stress conditions do not occur. The use of a transducer system requires addition of control apparatus to the hydraulic system.
Another prior art means for avoiding an over stress condition in the cylinder is by means of a relief valve in the cylinder or in the input line. This apparatus causes high heat build up and stress on the hydraulic pump.
A relief valve for a hydraulic piston is used in equipment manufactured by the Marathon Equipment Co. for compaction equipment, wherein a bypass valve member is positioned within a bore through the hydraulic piston such that the valve is displaced when its leading end engages the head gland of the cylinder. When the valve is displaced, a central reduced diameter portion of the valve spool comes into registry with a pair of axial passageways which communicate with the edge of the piston. In this design the hydraulic fluid is routed around the outside of the valve spool in a manner similar to typical manual control valves. The design requires special machining operations in the piston to provide fluid passageways. The machining required to create the passageways adds considerable expense to the fabrication of the piston. This previous design depends on extremely tight spool-to-bore clearance to minimize leakage in the closed position. The tight clearance makes the valve vulnerable to malfunction if there are machining inaccuracies in the piston or valve. Also the valve function becomes very sensitive to minute particles of contamination that could wedge between the spool and bore. As the outside diameter of the spool becomes worn, the hydraulic fluid leakage will increase, reducing the efficiency of the cylinder. Further, this prior design valve only works when the piston is advanced in one direction and it is found to tend toward premature exhaustion.
Another prior art device comprises a spring loaded relief valve positioned through the piston allowing a passageway for fluid to escape from the advancing side of the piston to the following side when the valve is urged against its spring loading by engagement of the leading end of the valve with the cylinder end wall. As with the Marathon Equipment Co. design valve, the bore through the piston requires complicated machining to provide valve seats and spring engaging shoulders. This valve structure causes heat build up, operates only in one direction of movement of the piston, and is subject to premature wear.